You know sites where you end up opening dozens of links of tabs and read through until you realize an entire Sunday is over?
TVtropes is the classic one but honestly that one doesn’t get me like it does some others. The bad ones for me are Cracked.com, Wikipedia, and a realtively new one for me, Buzzfeed.com.
How about you?
You tube can be like that for me. Especially if i’ve been drinking, and start watching Craig Tube videos. Example:
TVTropes, definitely - there’s so much cross-linking to other interesting content, it’s almost impossible to leave (and as a consequence, I often steer myself away completely before going there).
Oh, and UrbanDictionary - that can be quite sticky.
If, hypothetically speaking, there were porn sites, that showed pictures and links to pictures of naked people, I think that would qualify. But I doubt if there’s anything like that on the internet.
The straight dope. 
I spent hours reading all of the articles when I first stumbled upon it.
TVTropes and The Straight Dope are really the only ones I can think of, though sometimes a Wiki wander can be really time-consuming.
TVTropes has never done it for me. If someone links to something, I’ll click around, but the way the site is formatted, I get bored after a few minutes.
Youtube is awful, I can be on the site for hours watching all the related videos. Someone will link to a video about how to fold a shirt and 45 minutes later I’m learning how to give birth in a Russian Hospital with car alarms going off in the background or watching a 10 minute Friends blooper reel for the 5th time and is it really already 3am, I really should be in bed.
Wiki does the same thing to me, I can click away an entire day of work on that site.
Cracked.com and Bleacher Report. I’ll stay up til three am sometimes because I get so caught up reading different articles.
Youtube is a good one that I can’t believe I forgot in my OP. I remember I once spent hours watching videos about songs that sound like other songs.
I don’t Wiki wander. I live in Wikipedia, and occasionally wander out into the real world.
Sites where most pages have lots of links to related things (in some sense of the word) and where there’s no clear endpoint. So Cracked and most wikis, definitely, and blogs that are new to me or that I don’t go to very often, particularly ones that have been around a while.
Tvtropes, avclub and Cracked are the big ones for me. Agonybooth would be up there if I could figure out a way to see a list of only the text reviews. I don’t watch video reviews: it’s always 3 parts animated intro and some guy mugging for the camera and trying unsuccessfully to convince us that yelling and verbal tics are funny to every 1 part anything interesting about the movie/show/videogame being reviewed.
Amazon.com sometimes falls into this category for me.
Pinterest.
I have a bad habit of finding some webcomic and deciding I need to read each strip, starting with its initial posting in 2006.
Most recently has been Bug Comic with around 850 strips since it started in 2009. My supposed sleeping hours get filled with "Just one more… click… just one more… click… just one more…
When I first encountered the Savage Love column I lost a couple of days reading through the back issues.
Hubski has a lot of interesting links.
Youtube definitely. The Straight Dope I can “finish” reading without too much trouble if I avoid Great Debates, which I usually do.
My other big time sink is reddit.com.
The Chive.
Oof. That happens to me, too. Community bloopers or best-ofs of Whose Line do it for me.